The kidney is a highly vascularized organ that in the normal adult receives about 20% of the cardiac output. The unique spatial arrangement of the kidney arterioles with each nephron is crucial for the regulation of renal blood flow, glomerular filtration rate and other specialized kidney functions that maintain homeostasis. Thus, the proper and timely assembly ofthe arterioles with their respective nephrons is a crucial morphogenetic event leading to the formation of a functioning kidney necessary for independent extrauterine life. The mechanisms that govern the development ofthe kidney vasculature are poorly understood. Foxdl + cells and their descendants, the Ren+ precursors, are the earliest metanephric progenitors for all the mural cells ofthe kidney arterioles including JG cells, pericytes, arteriolar smooth muscle cells (SMCs) and mesangial cells. We recently showed that deletion of RBP-J (the final transcriptional effector for all Notch receptors) is required to maintain the number of renin-expressing cells, and that it is crucial in the plasticity of vascular SMCs to regain the renin phenotype in response to a threat to homeostasis. Preliminary lineage tracing studies in vivo also suggest that cells from the renin lineage harboring the RBP-J -/- mutation do not die, adopting instead a distinct myofibroblast phenotype. Those experiments suggest that RBP-J regulates the fate and maintenance of renin cells. We anticipate that similar mutation further upstream from the renin precursor, such as in the Foxdl + progenitors, will significantly affect the differentiation ofthe cells that compose the renal arteriolar tree.Using in vivo lineage tracing, time- and cell-specific conditional deletion approaches, genome wide epigenetic and gene-expression profiling and cell identification with appropriate differentiation markers we will test the he overall hypothesis that RBP-J is necessary for the differentiation of Foxdl + and Renin+ progenitor cells and the establishment of cell identity-specific epigenetic marks and gene-expression patterns that culminate with the emergence of the differentiated mural cells of the renal arterioles.